The current carrying capacity in PCB conductors is a common question from new designers. According to Rush PCB UK, along with the current carrying capacity of PCB conductors, the current carrying capacity of PCB vias is of equal importance, especially when the design is of a new board that must carry high currents. The designer must keep the conductor and via temperatures below an appropriate limit, and this in turn helps to keep components on the board cold enough.
Although the IPC 2152 standards deal extensively with the recommended current carrying capacity of traces, they focus much less on vias in multi-layered boards. However, there have been several investigations into current carrying capacity and temperature limits, while comparing them to the temperature excursions of typical traces carrying the same current.
Importance of Via Current Carrying Capacity
Designers typically specify the current carrying capacity of traces that will be carrying high current. They usually determine this using the copper weight and the allowable temperature rise, while using the nomograph in the IPC 2152 standards. They aim to size the traces such that the components and the board will remain within safe temperature limits while operating. If there are vias on the traces, it is important to compare the temperature rise in the vias with that of the traces and or planes to which they connect.
Figure 1: Various Types of PCB Vias
As the via connects, at each end, to a hot trace that is carrying a high current, it is reasonable to expect the via temperature to be at least as high as that of the traces that connect to it. Planes and traces connected to the via can get quite hot with the passage of high currents, especially for low copper weights carrying 5-10 Amperes. Therefore, it is natural to expect heat accumulation in vias. Also, with exposure to air for traces on the surface layers, it is natural to expect they will run cooler compared to traces buried in the interior layers of the board. These are significant considerations that relate to the via reliability, especially for microvias.
In reality, the situation is contrary to popular intuition. Traces on the surface run at temperatures higher than that of buried traces in internal layers. At 25 ℃, the thermal conductivity of air is approximately 0.026 W/mK, while that of FR-4 is about 0.25 W/mK. In addition, alternative substrate options are available that offer even higher thermal conductivity. That means the substrate acts more like a heat sink for conductors passing through it. As the substrate also surrounds vias, the above applies to vias as well. This helps to explain the fact that vias tend to have a lower temperature compared to traces that connect to them. An article in the Signal Integrity journal, by Douglas Brooks and Johannes Adam, substantiates the above through measurements and results.
Read About: Make Your PCBs More Reliable with Vias
Rule of Thumb
Engineers often prefer to follow the 0.5 A rule of thumb. As this rule offers a conservative result, it is acceptable to follow in most cases. However, for higher DC currents, an excessive number of vias may do more harm than good when the designer is connecting them to planes. For instance, the designer may have set a limit of 1 A per via. If they must supply 5 A instantaneously, they can safely place 5 large vias with thick plating, as long as the via temperature is not too high near a component.
In practice, the danger in the above example is not about the high temperature in the vias. Rather, it is more about temperature cycling. If the temperature swings between very low and very high temperatures, it might lead to fatigue setting in, resulting in failure.
Analysis of Current Carrying Capacity of Vias
In practice, thin traces on outer layers exposed to air and carrying high currents often operate at higher temperatures as compared to the temperature of vias connecting to them, although the difference is only a few degrees. This is due to the thermal conductivity of air being lower than the thermal conductivity of the substrate material surrounding the via. The net effect is the via loses heat faster than the thin traces can dissipate it into air.
While the above is true for thin traces, the situation reverses itself for wider traces. For instance, traces with widths of 200 mils and above can operate at temperatures lower than that of connected vias, although, the temperature difference is only a few degrees. This is because the heat loss from the wide trace now has two components. While the trace loses heat to the surrounding air, it also loses heat through the higher thermal conductivity of the substrate in contact with it. As the heat loss depends on surface area, and the trace is wide, the exposure is now through a much larger surface area of the trace. This allows a wide trace to have a lower temperature at equilibrium. Therefore, it is safe to summarize as follows:
- For thin traces, vias act as heat sinks for the trace
- For wide traces, the trace acts as a heat sink for the via
Of course, there are additional factors in the above analysis—the contribution from planes in layers. In reality, large planes act as additional heat sinks, further lowering the operating temperature of the conductors. Moreover, the designer can use alternative substrates with even higher thermal conductivity than that of FR-4. This removes more heat from conductors and vias, leading to an even lower temperature at equilibrium.
Deciding on Via Size
The designer must size the conductors according to the IPC 2152 standard guidelines for carrying high currents. They must also provide thick-walled vias. As the temperature of the via will not rise above that of its conductor, the design will not require further considerations related to vias.
The heat from the via will dissipate into the substrate and the nearby planes and traces. If the traces already have wide surfaces, they will dissipate more heat from their larger surfaces as compared to that dissipated by the via. Therefore, as the heat leaves the traces faster than it does from the via, the entire system will operate at a lower equilibrium temperature.
The current carrying capacity of the via also depends on its thermal conductivity, which the designer can control by adjusting the copper weight, thickness of the prepreg material, and/or the material filling the via.
Rush PCB UK recommends using a reputed PCB CAD design software that allows creating professional via designs and building a stackup from a wide variety of standard substrates.